Metal oxides such as titanium oxide (TiO.sub.2) have a function of decomposing organic compounds which are in contact therewith or present close thereto by oxidization or reduction when excited by ultraviolet rays, and thus are called photocatalytic semiconductors.
This function is well known and is utilized in a variety of applications. For example, Unexamined Japanese Patent Publication No. 62-66861 discloses an arrangement in which a photocatalytic element having a photocatalytic semiconductor applied to the surface of its base, which is in the form of a film, bead, board or fibrous element, is placed in a glass tube and gas or liquid is circulated through the glass tube. Also, Unexamined Japanese Patent Publication No. 1-143630 discloses an arrangement in which gas or liquid is circulated through a photocatalytic element having a photocatalytic semiconductor applied to the surface of its base which is in the form of a lattice, plate, particle, pleat or net. Further, Unexamined Japanese Patent Publication No. 4-61933 discloses an arrangement in which a photocatalytic semiconductor is applied directly to a stainless steel pipe or a porous fine tube.
In the arrangement of the conventional photocatalytic elements, the photocatalytic semiconductor is applied to a surface of a base which is a member (tube or plate) having a predetermined shape suitable for its use. In such cases, it is difficult to make a functional area of the photocatalytic semiconductor much greater than the surface area of the base. Further, in order for the photocatalytic semiconductor to be applied to the surface of the base, usually the photocatalytic semiconductor in a sol state is applied to the surface of the base and then the base with the photocatalytic semiconductor applied thereto is baked at a predetermined temperature (50 to 500.degree. C.). If, however, the base has a complicated shape, it may be difficult to make the photocatalytic semiconductor adhere to an inside surface of the base, or the base may be deformed due to heat. In addition, if the base is complicated in shape, it is difficult to irradiate its inside surface with ultraviolet rays.